The Ms point of austenitic-martensitic stainless steel is lower than room temperature, and it is an austenite structure after solution treatment, which is easy to form and weld. Generally, two kinds of process methods can be used to make it undergo martensitic transformation.
First, after the solution treatment, after heating at 700-800 degrees, the austenite is transformed into a metastable state due to the precipitation of chromium carbide, the Ms point rises above room temperature, and it transforms into martensite when cooled; second, directly after the solution treatment Cooling to between the Ms and Mf points, the austenite is transformed into martensite. The latter method can obtain higher corrosion resistance, but the interval from solution treatment to cryogenic should not be too long, otherwise the strengthening effect of cryogenic will be reduced due to the aging stabilization of austenite.
After the above treatment, the stainless steel is aged at 400-500 degrees to further strengthen the precipitated intermetallic compounds. Typical steel grades of this type of stainless steel are 17Cr-7Ni-A1, 15Cr9Ni-A1, 17Cr-5Ni-Mo, 15Cr-8Ni-Mo-A1 and so on.This type of stainless steel is also called austenitic-maraging stainless steel, and because in fact, in addition to austenite and martensite, there are different amounts of ferrite in the structure of these stainless steels, so they are also called semi-austenitic stainless steel. Precipitation hardening stainless steel.
This type of stainless steel is a new type of stainless steel developed and applied in the late 1950s. They are generally characterized by high strength (C can reach 100-150) and good thermal strength, but due to the low chromium content and the precipitation of chromium carbide during heat treatment.so the corrosion resistance is lower than that of standard austenitic stainless steels. It can also be said that the high strength of this type of steel is obtained by sacrificing part of the corrosion resistance and other properties (such as non-magnetic properties).
At present, this type of stainless steel is mainly used in the aviation industry and rocket and missile production, and is generally used in machinery manufacturing. They are not yet common, and they are also classified as a series of ultra-high strength steels.
